The recent trend toward weight reduction of automobiles for better fuel economy and improved safety from collisions has led to an increased demand for high-strength hot-rolled steel sheet. The hot-rolled steel sheet for such purposes is usually required to have both good elongation and good stretch flangeability from the standpoint of workability. In the present invention, it is expressed as having “excellent combined formability” which means that it excels in both elongation and stretch flangeability.
A high-strength hot-rolled steel sheet that is formed into parts of complex shape needs to meet requirements for high stretch flangeability when it undergoes stretch flanging and also for high elongation when it undergoes bulging simultaneously. Several methods are known in literature (as listed below) for improving elongation and flangeability separately. However, they have many problems unsolved yet.
Patent Document 1 discloses a steel sheet of bainitic-ferrite structure as a high-strength hot-rolled steel sheet to be formed. This steel sheet, however, is limited in tensile strength to 500 MPa level. Patent Document 2 discloses a steel sheet of bainite structure having a tensile strength exceeding 900 MPa level. This steel sheet, however, is not satisfactory despite its high strength with its total elongation being about 14% (as an index of workability) and its bore expanding ratio (λ) being about 40% (as an index of stretch flangeability).
Patent Document 3 discloses a steel sheet whose structure is composed of ferrite and retained austenite and whose strength is greater than 980 MPa. The steel sheet has a high elongation but is not necessarily satisfactory in stretch flangeability. Also, Patent Document 4 discloses a steel sheet whose structure is composed of ferrite and martensite or composed of ferrite, bainite, and martensite and whose strength is greater than 980 MPa. The steel sheet of such composite structure also exhibits a high elongation in its own way. However, nothing is mentioned about its stretch flangeability, and it will not express high stretch flangeability because it is based on a mixed structure composed of soft ferrite and hard martensite and bainite.
Patent Document 5 discloses a method for improving both strength and ductility by incorporating steel with copper in the state of atomic cluster. This method, however, does not provide a high strength comparable with that achieved by precipitation strengthening. In addition, the steel incorporated with copper exhibits a high strength of 980 MPa level but its bore expanding ratio (λ) as an index of local ductility is about 22% at the highest.
Patent Document 6 discloses a steel which has a composite structure of ferrite and bainite and which is modified by incorporation with copper. The steel, however, is not so high in strength due to insufficient copper added, and it is not intended to improve strength by utilizing the precipitation strengthening of copper.
Patent Document 7 discloses a hot-rolled steel sheet which has improved burring workability and fatigue characteristics by incorporation with copper and titanium. It is based on the idea that copper in the state of solid solution improves fatigue characteristics. The disclosed steel sheet, however, does not meet the requirements for both strength and workability.
Making parts in complex shape by a simple process needs a steel sheet with excellent combined forming performance, which excels in both elongation and stretch flangeability. It is not so difficult to impart such characteristic properties to a mild steel with a low strength. However, it is difficult to make a high-strength steel sheet possess both high elongation and high stretch flangeability (bore expanding ratio: λ). A steel sheet superior in one of these characteristic properties is inferior in the other. A probable reason for this is that elongation is related strongly with the structure of material; that is, a sample with a soft structure such as polygonal ferrite exhibits a high elongation but its stretch flangeability is affected intricately by structure uniformity and the size and distribution of precipitates and inclusions.
Patent Document 1:
Japanese Patent Laid-open No. Hei-6-172924
Patent Document 2:
Japanese Patent Laid-open No. Hei-11-80890
Patent Document 3:
Japanese Patent Laid-open No. 2000-290745
Patent Document 4:
Japanese Patent Laid-open No. 2003-73775
Patent Document 5:
Japanese Patent Laid-open No. 2003-73777
Patent Document 6:
Japanese Patent Laid-open No. 2003-55737
Patent Document 7:
Japanese Patent Laid-open No. 2001-200339